1. Field of the Invention
The present invention relates to an optical system, and more particularly to an optical system comprising a free curved reflective surface. The present invention also relates to a display apparatus, and more particularly to a display apparatus which is suited to be used as a head mounted type, which is called an HMD (head mounted display).
2. Description of Related Art
Japanese Patent No. 3155341 (reference 1) discloses a display apparatus which comprises an aspherical concave mirror of which curvature in the plane of incidence of the optical axis is set such that a plane image can be formed. Consequently, on the display, images of high picture quality can be seen.
U.S. Pat. No. 5,594,588 (reference 2) discloses a display apparatus which comprises an aspherical concave mirror of which curvature in a direction perpendicular to the plane of incidence of the optical axis is set such that distortion can be minimized. Consequently, on the display, images of high picture quality can be seen.
Japanese Patent Laid Open Publication No. 11-95160 (reference 3) discloses a head mounted display apparatus which comprises a half mirror and a polarizer which is laminated on the half mirror. Thereby, the quantity of reflected light is reduced.
However, the display apparatus of the reference 1 has a problem that correction of distortion is sacrificed for the sake of achieving a plane image. More specifically, as FIG. 17 shows, a rectangular image 500 is reflected on a free curved reflective surface 501 to be directed to an optical pupil 510, and a virtual image 505 is seen on the optical pupil 510 as a distorted image 505′.
Also, the display apparatus of the reference 2 has a problem that correction of curvature of field is sacrificed for the sake of minimizing distortion or a problem that resolution is low. More specifically, as FIG. 18 shows, a rectangular image 500 is reflected on a free curved reflective surface 502 to be directed to an optical pupil 510, and a virtual image 506 is seen on the optical pupil 510 as a rectangular image 506′. Distortion was corrected, and the image 506 is rectangular. However, the curvature of field of the image 506′ is large and more than 1 diopter, so that the image is difficult to see. With this free curved reflective surface 502, if it is tried to correct curvature of field as well as distortion, the image 506′ will have a low resolution and will be blurred.
In the display apparatus disclosed by the reference 3, a half mirror is used. Therefore, although the quantity of external light is reduced to a half, 25% of the quantity of light is reflected to be directed to the eyes of an observer, which makes the image difficult to see. Also, because only 25% of the quantity of light from the image is used, the image is dark. Further, because the light from the image is reflected by the half mirror, the apparatus is large.
Recently, various types of color image forming apparatuses have been developed and provided. Color image forming apparatuses are generally divided into a color filter type and a field sequential driving type.
FIG. 14a shows a screen on which images A and B are displayed. FIG. 14b schematically shows display elements of the color filter type, and FIG. 14c schematically shows display elements of the filed sequential driving type.
In the color filter type, a red color filter R, a green color filter B and a blue color filter B are provided for each pixel, and depending on the luminous balance of the three filters, a color image is formed. In this type of color image forming apparatus, because one dot is formed by use of three filters, a color shift in accordance with the pitch of the filters occur. The color shift is so small that it will not be a practical problem in an apparatus which enables an observer to see the formed image directly. However, in an apparatus which magnifies a formed image, such as an HMD, the color shift is also magnified, and the picture quality will be lowered.
On the other hand, in the filed sequential driving type, as shown by FIG. 14c and as disclosed by Japanese Patent Laid Open. Publication No. 2001-117045 (reference 4) and Japanese Patent Laid Open Publication No. 2001-188194 (reference 5), the illuminating light is switched among red, green and blue sequentially at a high speed, and synchronously, in each pixel, the light of red, the light of green and the light of blue are modulated in accordance with image signals of the respective wavelengths. Consequently, a color image can be seen by an after image effect. As a modulating device, an LCD (liquid crystal display), a DMD (digital micromirror device) made by U.S. Texas Instruments Incorporated or other suitable devices can be used.
In the field sequential driving type, an image of R, an image of G and an image of B are formed in one pixel sequentially, and a color shift does not occur. When the color filter type and the field. sequential driving type are to form images of the same resolution, the necessary number of pixels in the field sequential driving type is one third of that in the color filter type. When the color filter type and the field sequential driving type have the same displaying area, the size of each pixel of the filed sequential driving type is three times as large as that of the color filter type, and the field sequential driving type has a higher vignetting factor and can form brighter images.
In an optical system for magnifying a formed image and displaying the magnified image, an optical element which diverts a bundle of rays is necessary, and a refraction element or a reflection element is used. With respect to diversions of bundles of rays by a refraction element, as FIG. 15 shows, a medium refracts a bundle of red rays R, a bundle of green rays G and a bundle of blue rays B at different angles because the medium has different refractive indexes to the respective wavelengths of R, G and B. Thereby, chromatic aberration is caused.
On the other hand, with respect to bends of bundles of rays by a reflection element, as FIG. 16 shows, a reflective element reflects a bundle of red rays R, a bundle of green rays G and a bundle of blue rays B at the same angle, and chromatic aberration is not caused. Japanese Patent Laid Open Publication No. 5-303054 (reference 6) discloses a magnifying optical system which uses a reflective surface with this characteristic. However, the reference 6 merely discloses the magnifying optical system.